The present invention relates to fiber-optic communications networks, and more particularly, to optical amplifier arrangements with transient control capabilities for use in optical communications networks.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been subject to attenuation over fiber-optic links. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers. Raman amplifiers have also been investigated. Discrete Raman amplifiers may use coils of dispersion-compensating fiber to provide Raman gain. Distributed Raman amplifiers provide gain in the transmission fiber spans that are used to carry optical data signals between network nodes.
Sometimes channels in a communications link may be abruptly added or dropped due to a network reconfiguration. Channels may sometimes be dropped due to accidental fiber cuts. When the number of channels carried by a link changes abruptly, the total signal power being transported over the link changes suddenly. This can adversely affect the performance of distributed or discrete amplifier equipment in the link.
For example, if a distributed Raman amplifier is pumped at a constant power, these sudden changes in signal power will result in transient effects in the gain of the distributed Raman amplifier. Gain transients in a distributed Raman amplifier may cause fluctuations in the power of the output signals at the end of a fiber span. Output signals that are too weak may be difficult to detect without errors. Output signals that are too strong may give rise to nonlinear optical effects.
Abrupt changes in the signal power on the link may also cause undesirable transient effects in the gain of a discrete Raman amplifier or an erbium-doped fiber amplifier.
It is an object of the present invention to provide optical amplifier arrangements for fiber-optic communications system with transient control capabilities.
It is also an object of the present invention to provide optical amplifier arrangements in which gain transients are controlled using information on the optical power of a particular channel or channels in an optical communications link.
These and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifier arrangements for wavelength-division-multiplexing optical communications links that support multiple channels operating at different wavelengths. Optical amplifier equipment may be used to amplify optical signals on a given communications link. The optical amplifier equipment may be based on distributed or discrete Raman amplifiers, rare-earth-doped fiber amplifiers such as erbium-doped fiber amplifiers, or any other suitable amplifiers.
The optical signals on the link may be monitored using optical monitoring equipment. The optical monitoring equipment may include one or more filters for filtering out the optical signals corresponding to a particular channel or channels from the other optical signals. The optical power for this particular channel or channels may be maintained at a constant level by controlling the optical amplifier equipment. For example, the optical power for the particular channel or channels may be maintained at a constant level by adjusting the pump power in the amplifier equipment to maintain the gain of the optical amplifier at a constant level. The channel or channels that are monitored may be channels that the network provider ensures will always be present during normal operation of the network. Controlling the amplifier equipment in this way suppresses gain transients for the other channels, even when some of those other channels are added or dropped abruptly.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.